August 2002

Audio Research VS-55 Stereo Amplifier: Measurements

All amplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test amplifiers there. All measurement data and graphical information displayed below are the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

• Measurements were made with 120V AC line voltage.
• Output tube plate current adjusted to 65mA per tube when warmed up.
• Power output and distortion plotted with both channels driven.
• Gain: 18.3x, 25.2dB.
• Output noise, 8-ohm load, 1k-ohm input termination: wideband 0.40mV, -77.0dBW; A weighted 0.088mV, -90.1dBW.
• AC line current draw at idle: 1.9A.
• Output impedance at 50Hz: 0.89 ohms.
• This amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 50W
• 8-ohm load at 10% THD: 58W
  
  
• 4-ohm load at 1% THD: 33W
• 4-ohm load at 10% THD: 70W
  
  
• 16-ohm load at 1% THD: 33W
• 16-ohm load at 10% THD: 38W

General

Measurement results for this design are typical of other well designed similar powered units. As can be seen in Chart 1, output regulation with load change is reasonable and the response variation with the NHT dummy speaker load is within +/- 0.9dB over the audio range. High-frequency bandwidth with a resistive load is about 50kHz. The tendency to peak with an open circuit load could possibly give a slight high frequency rise above 10kHz with speakers with a high frequency increasing impedance load -- typical of most dome tweeters. Harmonic distortion is reasonably low in the critical "first watt" region for 8-ohm and higher loading on the 8-ohm tap, but does increase with 4-ohm loading. On the other hand, 4-ohm loading on the 8-ohm tap does increase the power output at onset of clipping from 50 to 60 watts. While the 16-ohm power on the 8-ohm tap is down to about 28 watts, the overall matching of the output transformer to favor some power increase with low loading on an output tap is a good choice. The amount of distortion rise with frequency in Chart 3 is quite typical of many power amplifiers, both tube and solid state. In the spectrum of harmonic distortion residue of a 1kHz signal at the 10W power level in Chart 5, the odd harmonics are dominant with the even harmonics quite a bit lower indicating good push-pull balance. Although the magnitude of the AC line harmonics is similar to other amplifiers, the noise floor is quite low between the harmonics. Of interest, and technically not desirable, is the presence of a number of 120Hz spaced sidebands about the nulled out fundamental 1kHz signal. The damping factor in Chart 4 stays up at low frequencies but does start to decrease above 1kHz, again typical of many power amplifiers both tube and solid state.

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
Green line: NHT dummy-speaker load

(line up at 20W to determine lines)
Top line: 8-ohm SMPTE IM
Second line: 4-ohm on 8-ohm tap THD+N
Third line: 16-ohm on 8-ohm tap THD+N
Bottom line: 8-ohm on 8-ohm tap THD+N

8-ohm output loading on 8-ohm tap
Green line: 50W
Cyan line: 30W
Blue line: 10W
Red line: 1W

Damping factor = output impedance divided into 8

1kHz signal at 10W into an 8-ohm load

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